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Full Discussion: Writing a service in Linux
Top Forums UNIX for Advanced & Expert Users Writing a service in Linux Post 30661 by lusername on Thursday 24th of October 2002 10:02:30 PM
Old 10-24-2002
If your program is written in C, your program should use the daemon() library call to make your program a daemon. This will cause your program to close its standard input, output, and error, and fork() into the background.

You should then add a command to one of your boot scripts to cause your program to run. The location of the script to change differs between Linux distributions. If you are running Red Hat, you should edit /etc/rc.d/rc.local. The script you need to edit is started directly or indirectly from /etc/inittab.

#include <unistd.h>
#include <err.h>

int main()
   if(daemon(0,0) == -1)
       err(1, NULL);

      /* do stuff */

# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.

# Run my-daemon in the background

Your daemon will run until the system shuts down, or until it crashes. Also, if your daemon has a memory leak, the kernel will kill it when it uses too much RAM.

If you make your program a non-daemon that closes its standard input/output/err, you can start it directly from inittab, with the action field set to "respawn". In this case, if your program crashes, init will restart it.

#include <unistd.h>

int main()

      /* do stuff */

# inittab       This file describes how the INIT process should set up
#               the system in a certain run-level.

# [ snip for brevity ]

# Run only in runlevel 5. Init will kill my-service if
# another runlevel is selected. If it crashes quickly
# and repeatedly, init will stop restarting it for 5 minutes.


Test Your Knowledge in Computers #990
Difficulty: Medium
UNIX System V is one of the first commercial versions of the Unix operating system and was originally developed by AT&T and first released in 1981.
True or False?

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INITTAB(5)                                              Linux System Administrator's Manual                                             INITTAB(5)

inittab - format of the inittab file used by the sysv-compatible init process DESCRIPTION
The inittab file describes which processes are started at bootup and during normal operation (e.g. /etc/init.d/boot, /etc/init.d/rc, get- tys...). Init(8) distinguishes multiple runlevels, each of which can have its own set of processes that are started. Valid runlevels are 0-6 plus A, B, and C for ondemand entries. An entry in the inittab file has the following format: id:runlevels:action:process Lines beginning with `#' are ignored. id is a unique sequence of 1-4 characters which identifies an entry in inittab (for versions of sysvinit compiled with the old libc5 (< 5.2.18) or a.out libraries the limit is 2 characters). Note: traditionally, for getty and other login processes, the value of the id field is kept the same as the suffix of the corre- sponding tty, e.g. 1 for tty1. Some ancient login accounting programs might expect this, though I can't think of any. runlevels lists the runlevels for which the specified action should be taken. action describes which action should be taken. process specifies the process to be executed. If the process field starts with a `+' character, init will not do utmp and wtmp accounting for that process. This is needed for gettys that insist on doing their own utmp/wtmp housekeeping. This is also a historic bug. The runlevels field may contain multiple characters for different runlevels. For example, 123 specifies that the process should be started in runlevels 1, 2, and 3. The runlevels for ondemand entries may contain an A, B, or C. The runlevels field of sysinit, boot, and boot- wait entries are ignored. When the system runlevel is changed, any running processes that are not specified for the new runlevel are killed, first with SIGTERM, then with SIGKILL. Valid actions for the action field are: respawn The process will be restarted whenever it terminates (e.g. getty). wait The process will be started once when the specified runlevel is entered and init will wait for its termination. once The process will be executed once when the specified runlevel is entered. boot The process will be executed during system boot. The runlevels field is ignored. bootwait The process will be executed during system boot, while init waits for its termination (e.g. /etc/rc). The runlevels field is ignored. off This does nothing. ondemand A process marked with an ondemand runlevel will be executed whenever the specified ondemand runlevel is called. However, no run- level change will occur (ondemand runlevels are `a', `b', and `c'). initdefault An initdefault entry specifies the runlevel which should be entered after system boot. If none exists, init will ask for a runlevel on the console. The process field is ignored. sysinit The process will be executed during system boot. It will be executed before any boot or bootwait entries. The runlevels field is ignored. powerwait The process will be executed when the power goes down. Init is usually informed about this by a process talking to a UPS connected to the computer. Init will wait for the process to finish before continuing. powerfail As for powerwait, except that init does not wait for the process's completion. powerokwait This process will be executed as soon as init is informed that the power has been restored. powerfailnow This process will be executed when init is told that the battery of the external UPS is almost empty and the power is failing (pro- vided that the external UPS and the monitoring process are able to detect this condition). ctrlaltdel The process will be executed when init receives the SIGINT signal. This means that someone on the system console has pressed the CTRL-ALT-DEL key combination. Typically one wants to execute some sort of shutdown either to get into single-user level or to reboot the machine. kbrequest The process will be executed when init receives a signal from the keyboard handler that a special key combination was pressed on the console keyboard. The documentation for this function is not complete yet; more documentation can be found in the kbd-x.xx packages (most recent was kbd-0.94 at the time of this writing). Basically you want to map some keyboard combination to the "KeyboardSignal" action. For exam- ple, to map Alt-Uparrow for this purpose use the following in your keymaps file: alt keycode 103 = KeyboardSignal EXAMPLES
This is an example of a inittab which resembles the old Linux inittab: # inittab for linux id:1:initdefault: rc::bootwait:/etc/rc 1:1:respawn:/etc/getty 9600 tty1 2:1:respawn:/etc/getty 9600 tty2 3:1:respawn:/etc/getty 9600 tty3 4:1:respawn:/etc/getty 9600 tty4 This inittab file executes /etc/rc during boot and starts gettys on tty1-tty4. A more elaborate inittab with different runlevels (see the comments inside): # Level to run in id:2:initdefault: # Boot-time system configuration/initialization script. si::sysinit:/etc/init.d/rcS # What to do in single-user mode. ~:S:wait:/sbin/sulogin # /etc/init.d executes the S and K scripts upon change # of runlevel. # # Runlevel 0 is halt. # Runlevel 1 is single-user. # Runlevels 2-5 are multi-user. # Runlevel 6 is reboot. l0:0:wait:/etc/init.d/rc 0 l1:1:wait:/etc/init.d/rc 1 l2:2:wait:/etc/init.d/rc 2 l3:3:wait:/etc/init.d/rc 3 l4:4:wait:/etc/init.d/rc 4 l5:5:wait:/etc/init.d/rc 5 l6:6:wait:/etc/init.d/rc 6 # What to do at the "3 finger salute". ca::ctrlaltdel:/sbin/shutdown -t1 -h now # Runlevel 2,3: getty on virtual consoles # Runlevel 3: getty on terminal (ttyS0) and modem (ttyS1) 1:23:respawn:/sbin/getty tty1 VC linux 2:23:respawn:/sbin/getty tty2 VC linux 3:23:respawn:/sbin/getty tty3 VC linux 4:23:respawn:/sbin/getty tty4 VC linux S0:3:respawn:/sbin/getty -L 9600 ttyS0 vt320 S1:3:respawn:/sbin/mgetty -x0 -D ttyS1 FILES
/etc/inittab AUTHOR
Init was written by Miquel van Smoorenburg ( This manual page was written by Sebastian Lederer (lederer@fran- and modified by Michael Haardt ( SEE ALSO
init(8), telinit(8) Dec 4, 2001 INITTAB(5)

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